I am about to lose my mind. Here's the story. I have an 02' Boxster S. I fill the tank, and it gets completely full... of course, only after manipulating the stupid pump handle to get the tank full as it shuts off every 2 to 3 gallons.... but I digress. I drive the car thereafter, and it runs perfect! Until.... just below 3/4 tank... should be approximately 4 gallons remaining. And then the car acts like it has freakin' water in the gas.... it quits, or stumbles, and shuts off for a moment, only to catch again... I though...ahhhh, I got water in my gas. So I put some fuel treatment into the tank that is the anti freeze/remove H2O from you tank stuff... I've used this in the past on various hoopties I've owned in the poorer days.... haha. I ran a couple tanks this way, being sure not to get below 3/4 tank.... and today was the day off to experiment by driving below 3/4 tank between to strategically located gas stations. Oh, I forgot to mention that when this crap happens... just a single gallon of gas, and the car runs like new again.... anyway... today, at just a tad under the 3/4 tank level... the car freakin' died, and absolutely would not restart until I put 2 gallons of gas into the tank... than like new again. I called someone that has worked on my car before... too far to drive at this point to be practical.... and he suggested maybe the fuel filter was clogged or the fuel pump was on the way out. I just finished searching every Boxster forum I could find, and the impending signs of fuel pump failure are nothing like this at all... But putting in a fuel additive does nothing! HELP!!!! I'm going to sit here and drink Sam Adams Cherry Wheat until someone replies! I'm going nutz! Thanks in advance.

Stupid question for my own understanding:
You write that at 3/4 there should 4 gallons remaining
The Boxster has a 16 gallon tank....

At 3/4 GONE, there should be approximately 4 gal. remaining. Let's please stick to the topic, and not argue semantics.... especially when I already admitted I am well into my 6 pack of Sammies. Thanks.

I replaced the gas cap shortly after purchasing the car as the o ring inside the gas cap was dry drotted... car runs perfect so long as I don't go below 3/4 of a tank of gas.... it was the dry rot on the gas cap that lead me to believe that there was likely water in the gas tank. .....

Hmmmm.... maybe I need to supply more specific information.... gas in the tank. completely full. drive... until only 1/4 tank remains. than engine dies, until I add a minimum of 1 gallon of gas... than runs perfect.... like a stallion. changed the gas cap shortly after buying the car... this problem existed since I bought the car... so the problem was before AND after I changed the gas cap... just won't run at all when the gas level is below 1/4th tank.

Wow, pretty snippy for someone asking for help....
Getting the "semantics" straight, helps us help you

It may be a venting issue since you say you have issues getting gas into the car. I had a similar issue and had to have the flapper valve in my gas tank filler neck modified. There is a TSB about this issue.

Or you have a tank fuel pickup issue...sounds as if the pickup has been raised in the tank creating what would be similar to being out of gas. Perhaps it has come loose or was repositioned somehow incorrectly.

__________________DanGGR AX and TT instructor for 20+ yrs.Maintained by Me